The present invention relates to an apparatus for use in seismic surveying.
Seismic surveying, or reflection seismology, is used to map the Earth's subsurface. A controlled seismic source provides a low frequency seismic wave which travels through the subsurface of the Earth. At interfaces between dissimilar rock layers, the seismic wave is partially reflected. The reflected waves return to the surface where they are detected by one or more of seismic sensors. Ground vibration detected at the earth surface can have a very wide dynamic range, with particle displacement distances ranging from centimetres to angstroms. Data recorded by the sensors is analysed to reveal the structure and composition of the sub surface.
Seismic sensors (also known as seismometers or geophones) typically comprise an electric coil of wire immersed in a strong magnetic field. These electromagnetic sensors can be constructed as either moving magnet or moving coil types, with the latter exclusively favoured for seismic exploration. In the moving coil version, the magnet is fixed to the case, which is then firmly planted in the earth, such that the case and magnet move in unison with ground displacements. The moving electrical coil is immersed in the magnetic field gap of a fixed magnet and the coil is loosely coupled to the geophone case by soft springs in such a way that the coil can only travel along a single axis. As the coil moves along this axis, relative to the fixed magnet, it will progressively cut through lines of magnetic flux, and generate a voltage and current at the electrical terminals of the coil, in proportion to the velocity of ground displacement. In the moving coil type, the coil forms the proof or reaction mass.
The coil and spring arrangement will have a resonant frequency dependant on the mass of the coil and the compliance of the springs. At frequencies well below resonance, the coil and magnet move in unison so that sensitivity is low and voltage or current output is small. As the frequency of vibrations increase upwards to and beyond the resonant frequency of the geophone the sensitivity and output increase, peak and flatten respectively. Typically the resonant frequency of geophones falls in the 10 to 30 Hz range, preference given to the low end. Low frequency resonance requires high spring compliance. This subsequently requires soft springs which in turn mandate precise design and construction of the sensor to achieve the required sensitivity, robustness, linearity and immunity to off axis distortion, necessary for seismic acquisition. The design tradeoffs among field strength, magnet size/weight, coil geometry and spring compliance are critical to design and construction of a geophone which has sufficient sensitivity, voltage-current output, linearity and robustness to simultaneously measure both the large and small surface vibrations described above.
A further type of geophone in use for seismic surveying uses capacitance to generate the electrical signal. These are typically constructed as Microelectromechanical systems (MEMS) using micro machined silicon with metal plating applied to facing components on either side of a small plated and spring loaded proof mass. These MEMS sensors may have the advantage of small size and weight compared to a moving coil geophone. The movement of the MEMS proof mass relative to the outer fixed plates creates variable capacitance which can be detected as a vibrations signal proportional to the acceleration of the sensor displacement. The springs are formed from regions of thin cut silicon, allowing a small linear displacement, and resonant frequencies above 1 kHz. The small capacitive surface area, high resonant frequency and restricted limit of linear travel mean that sensitivity will be quite low compared to a moving coil geophone. To counter this specialized electronics is used to hold the MEMS geophone in force feedback state. This required additional electronic circuitry requires space and power and partially defeats the MEMS advantages of size and weight compared to the passive moving coil geophone.
The term proof mass may also be known as a reaction mass or seismic mass and is generally known in the art to refer to a mass in a seismic sensor which moves as a consequence of the earth moving during a seismic acquisition survey.